1. Field of the Invention
The present invention generally relates to the treatment of liquids stored in tanks and to methods of agitating and stirring liquids.
More specifically, the invention relates to a method of treating a body of liquid in a tank, to a method of operating a process plant, and to a process plant.
2. Description of the Related Art
In the art of treating liquids, measures for stirring or agitating the liquid frequently are of major importance, such stirring serving purposes such as homogenization, i.e. equalization of differences in concentration and temperature, intensification of heat transfer between the liquid and a heat exchange surface, suspension of dissolution of a solid in the liquid, dispersion of immiscible liquids or sparging of a gas in the liquid. Particular fields of application within biotechnology industries comprise beer fermenters or yeast tanks, wherein mixing is applied in order to obtain uniformity in concentration of ingredients and temperature. Other fields of application are the processing of foods or cosmetics wherein there is a need for mixing minute quantities of ingredients into large volumes of matter. Other fields of application are found in the pulp and paper industry and within general chemical industries related to processes for the preparation of paints, of polymers, of drilling muds and others.
Mixing operations are often performed in vessels provided with various agitation means such as rotary impellers or jets. Baffles are typically provided for serving the purpose of preventing bulk rotation or swirling of the contents in the tank due to the effect of a rotating impeller or similar. The baffles sometimes used to prevent swirl or vortex formation represent a structural complication and also an operational complication due to the creation of dead volumes and due to the shadowing of surfaces that complicates cleaning.
Rotary impellers require drive motors and structural support for bearings as well as for the motors. A rotary impeller for a large tank typically comprises a rotary shaft with several stages of impellers. The rotary shaft is typically supported by bearings in both ends as well as by bearings intermediate the ends. Rotary impellers are often incorporated in closed vessels wherein the shaft penetrates the wall of the vessel. The impeller blades, the bearings and the supporting structure all adds to the complications in cleaning due to the extra surfaces and due to shading effects.
Cleaning is another basic process in process plants, which in general terms fulfils the purpose of removing residues for a variety of purposes such as for avoiding cross contamination, avoiding build up of barrier layers, and preparing the respective part of the plant for another batch of product, whether of a similar type or of a different type.
U.S. Pat. No. 5,620,250 teaches a jet mixer comprising a rotary impeller driven in rotation by the thrust from jets arranged at the tips of the impeller blades. Rotation is driven by the introduction of fluid, which fluid also provides a bearing between a body portion and the impeller. The stated object of this device is to provide mixing without a requirement for a motor and a gearbox with seals.
A mixer of this kind suffers certain restrictions in fields of application. Driving the rotary impeller by the reaction force created by the jets makes the speed of revolution highly variable depending on the fluid pressure applied and the viscosity of the body of liquid in the tank. Further, an impeller rotating about a fixed axis is bound to create a steady pattern of circulation in the tank that may leave dead volumes or volumes with low rates of agitation, in particular in case the tank is provided with internal structural elements as may often be the case. Agitation by an impeller rotating about a fixed axis is likely to create a swirl or a vortex inside the tank that may have to be countered by additional measures. Further, the impeller gives rise to shading that complicates cleaning of the tank inside, and the impeller by itself introduces surfaces that may need special precautions in the cleaning.
U.S. Pat. No. 4,166,704 discloses a rotating fluid jet agitator for mixing, comprising primary jet mixing nozzles arranged to rotate about a vertical axis and adapted to project streams of liquid along the plane of rotation, and a drive nozzle arranged to deliver a thrust which causes the primary nozzles to rotate about the vertical axis. The drive nozzle may be adapted for rotation about a horizontal axis, in which case the drive nozzle is structurally connected to a drag plate which is rotated due to the force of gravity balanced by the drag created in rotation, thus controlling the angle of the drive nozzle and thereby the net torque applied by the drive nozzle to the rotation of the primary nozzles.
Thus the primary nozzles rotate about a fixed vertical axis, whereas the drive nozzle may oscillate about a horizontal axis in a way with no positive control.
Rotating the jets by the reaction force from one jet is likely to be variable with respect to speed depending on factors such as drive fluid pressure and viscosity of the bulk of liquid inside the tank. Agitation by the effect of the jets from nozzles rotating about a stationary vertical axis is likely to create a fairly steady pattern of agitation that may leave a dead volume, e.g. in parts of the tank volume far away from the plane of rotation of the primary nozzles.
U.S. Pat. No. 5,810,473 discloses a liquid jetting device having a nozzle, which device is provided with separate power sources for swinging the nozzle in the vertical and horizontal directions. The jetting device is adapted for installation at the sidewall or at the top of a large oil tank for serving the purpose of fluidizing petroleum sludge to prevent the precipitation of sludge or to remove deposits on the tank floor. The nozzle injects high-pressure fluid provided by withdrawing a flow of liquid from the contents of the tank. Swinging of the nozzle is confined to stay within sectors of angles only.
The drive mechanism for swinging the nozzle comprises a complicated set of gears and end-stop switches and a set of seals and packings for allowing the shafts to cross the barrier to the high-pressure liquid.
An apparatus of this kind is subject to certain constraints. Swinging of the nozzle about two perpendicular axes by respective power drives does not inherently guarantee evenly distributed coverage of the volume by the jets. Agitation of the liquid by a wall mounted nozzle device is likely to create a net reaction force onto the nozzle base that strains the structure. Inside the body of liquid, agitation by a wall mounted nozzle is quite likely to create a rather steady pattern of flow, e.g. a swirl or another pattern that may leave dead volumes.
U.S. Pat. No. 5,333,630 discloses an apparatus for the cleaning of a closed compartment, which apparatus comprises a hub with nozzles arranged for rotation in a lower housing portion about a horizontal axis, which lower housing portion is again arranged for rotation about a vertical axis. The nozzle head is provided with a turbine and with gears for effecting rotation about both of these axes in order that the nozzles during rotation can sweep the whole of the interior of the closed compartment. This apparatus is adapted for cleaning compartments by means of sprayed out liquids. The gears are rinsed through by the liquid and the apparatus comprises slots and openings for allowing liquid to flow out in order to sweep the outside of the housing so as to make the apparatus self-cleaning.
Within this context the tank may comprise any container or generally closed envelope or structure adapted for generally enclosing a volume of liquid. Examples of such envelopes comprise any kind of tanks, containers, conduits or pipes. The dwell time of the liquid may range from seconds, as in the case of a pipeline that serves primarily for transportation, to days, months or years, as in the case of a container that serves primarily for storage purposes.